1. Field of the Invention
The present invention relates to a developing device in which an electrostatic latent image formed on an image carrier is converted into a visible image in an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus.
2. Description of the Related Art
The two-component developing method is well known. In the two-component developing device, a two-component developing agent is used. The two-component developing agent comprises a mixture of coloring particles, called toner particles, and magnetic particles, called carrier particles. The toner particles are adhered to an electrostatic latent image formed on an image carrier by electrostatic force. By this means, the electrostatic latent image is converted into a visible image.
However, this two-component developing method has problems, in that there is a tendency for the developing device itself to be larger, and also that fine adjustment of the mixture ratio of the toner particles and carrier particles is necessary. Therefore, a one-component developing method which does not require carrier particles is now most often used in copiers and compact printers.
An impression development method is one type of one-component developing method. This method is characterized in that the transporting of toner particles charged by friction to the developing area is achieved by causing toner particles to adhere to the surface of a developing roller. Since no magnetic carrier particles are required, there are many advantages; for example, simplification and miniaturization of the apparatus, and easy use of color toner particles.
In this imression development method, the toner particles adhering to the developing roller are formed into a thin layer. The thin layer of toner particles is made by controlling the amount of toner particles transported using a blade, the blade being in pressure contact with the developing roller. After this, the latent image is rendered visible by bringing the developing roller close to or into pressing contact with the image carrier. The blade is formed of a thin spring plate. As shown in Japanese Patent Disclosure (Kokai) No. 1-191878 (Disclosed on Aug. 1, 1989), on the leading edge of the blade, a curved portion, with a semicircular cross-sectional shape, is formed. By making the cross-section of curved portion semicircular in this way, a toner collecting space is formed between the curved portion and the surface of developing roller. The design is such that the curved portion is pushed upward by the collection of toner particles in the space so that approximately the specified quantity of toner particles passes between the developing roller and the curved portion.
However, in this type of developing device with the blade having a semicircular cross-section curved portion, over a long period of use the toner collecting space becomes shallower, due to wear of the surface of curved portion. Therefore the quantity of toner particles passing between the developing roller and the curved portion is reduced, due to reduction of upward pressure on the curved portion. As a result, deviations from the correct values of the toner charge and layer thickness occur. Therefore, the problem arises of not being able to obtain a high quality image.